1. Field of the Invention
The present invention relates to a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for ranging for UpLink (UL) resource allocation in an Orthogonal Frequency Division Multiple Access (OFDMA) system.
2. Description of the Related Art
A wireless access scheme of an Orthogonal Frequency Division Multiple Access (OFDMA) broadband wireless communication system is being standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.16/16e standardization group.
The broadband wireless communication system performs a ranging process in order to set accurate time/frequency offsets between a Base Station (BS) and a Mobile Station (MS) and to correct a Transmit (TX) power of a mobile station. The ranging process may be classified into initial ranging, handover ranging, periodic ranging, and bandwidth request ranging according to a purpose of the ranging.
The broadband wireless communication system classifies Code Division Multiple Access (CDMA) ranging codes according a purpose of the ranging by the mobile station as illustrated in Table 1 below.
Table 1 illustrates ranging codes used in the IEEE 802.16 system.
TABLE 1NameType(1 byte)LengthValueStart of ranging1551Indicates the starting number, S, of the groupcodes groupof codes used for this UpLink (UL). If notspecified, the default value shall be set tozero. All the ranging codes used on this ULshall be between S and ((S + O + N + M + L) mod256) whereN is the number of initial ranging codesM is the number of periodic ranging codesL is the number of bandwidth request rangingcodesO is the number of handover ranging codesThe range of values is 0 ≦ S ≦ 255.Handover1941Number of handover ranging CDMA codes.Ranging CodesPossible values are 0-255.Initial ranging1501Number of initial ranging CDMA codes.codesPossible values are 0-255.Periodic ranging1511Number of periodic ranging CDMA codes.codesPossible values are 0-255.Bandwidth1521Number of bandwidth request ranging codes.request codesPossible values are 0-255.
As illustrated in Table 1, the broadband wireless communication system classifies CDMA codes into an initial ranging code, a handover ranging code, a periodic ranging code, and a bandwidth request code according to a purpose of the ranging.
Accordingly, in order to perform ranging, the mobile station selects a code corresponding to a purpose of the ranging, among the CDMA codes negotiated with the base station, and transmits the selected code to the base station through a ranging region of an uplink subframe.
FIG. 1 is a diagram illustrating a general uplink subframe according to the related art.
Referring to FIG. 1, the uplink subframe is allocated a ranging region for transmitting a ranging code of the mobile station. The ranging region includes an initial ranging region 101 for transmitting the initial ranging code or the handover ranging code and a periodic ranging region 103 for transmitting the periodic ranging code or the bandwidth request code.
In a broadband wireless communication system of the related art, when a mobile station uses an initial ranging region 101 to transmit an initial ranging code or a handover ranging code to a base station, the base station allocates the mobile station a predefined amount of bandwidth through a CDMA Allocation Information Element (CDMA_Allocation_IE). That is, the base station allocates an uplink resource in consideration the maximum size of a RaNGing REQuest (RNG-REQ) message because the base station does not know how much data a CDMA code received from the mobile station will utilize. The bandwidth allocated to the mobile station by the base station is about 60 bytes corresponding to two or more subchannels. That is, the broadband wireless communication system of the related art uses an uplink resource of more than two subchannels in order to perform uplink transmission of the mobile station in a ranging process, thus degrading a base station cell-boundary reception rate and limiting the base station coverage area.
Thus, an uplink resource of less than one subchannel is allocated in a ranging process in the broadband wireless communication system of the related art. However, in the broadband wireless communication system of the related art, the mobile station and the base station are difficult to change because they are implemented on the basis of the IEEE 802.16/16e standards. Also, because the number of CDMA codes available to the base station is limited, it is difficult to add a new code group. Also, the addition of a new code group reduces the number of CDMA codes within the code group, thus increasing the probability of a collision between the CDMA codes.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.